Following ureteral obstruction - as seen in patients with ureteral calculi, ureteral stricture, and congenital ureteral obstruction, there is a significant alteration in renal hemodynamics and tubular function. Obstructive nephropathy remains a leading cause of end-stage renal diseases in both adult and pediatric populations requiring dialysis and renal transplantation. Furthermore, cbxonic obstruction can lead to a severe alteration in tubular function, such as nephrogenic diabetes insipidus, resulting in polyuria and dehydration. The molecular mechanisms underlying these changes have been investigated intensely during the last few decades. In particular, the role of prostaglandin (PG) in response to ureteral obstruction has been studied quite extensively. One of the key rate-limiting regulatory steps in PG-biosynthesis occurs at cyclooxygenase (COX), the enzyme complex that converts arachidonic acid into a common intermediate precursor, prostaglandin H2 (PGH2). Depending on the presence of specific synthases, PGH2 is rapidly converted into various prostanoids. It has been demonstrated that two distinct COX isoforms exist in mammalian tissues, each with differential patterns of regulation and biological functions. Both COX-1 and COX-2 have been described in the kidney with unique expression and function. We have demonstrated recently that bilateral ureteral obstruction stimulates the expression of inducible isoform, COX-2, in the inner medullary collecting duct cells, the cell type which is normally devoid of COX-2 expression. We also obtained preliminary evidence that this obstruction induced COX-2 in the collecting duct cells might involve intracellular signaling via reactive oxygen species upon mechanical stimulation. These findings led us to hypothesize that COX-2 may play an important regulatory role in the alteration of collecting duct cell physiology after ureteral obstruction. Defining the role and the regulatory mechanism of COX-2 expression in collecting duct cells after obstruction should lead to the development of novel therapeutic strategies employing COX-2 specific inhibitors in the treatment of obstructive nephropathy. In this proposal, we will study the following: 1. Does COX-2 mediate obstruction-induced water loss by altering the expression of AQP2 in the collecting duct cells? 2. What is the role of reactive oxygen species (ROS) in the regulation of COX-2 expression in the collecting duct cells after obstruction and stretch stimulation? 3. What are the down-stream target genes of obstruction-induced COX-2 in the collecting duct cells?